Rotatable storage media devices, such as magnetic disk drives and optical disk drives, are an integral part of computers and other devices with needs for large amounts of reliable memory. Rotatable storage media devices are inexpensive, relatively easy to manufacture, forgiving where manufacturing flaws are present, and capable of storing large amounts of information in relatively small spaces.
A typical device having a rotatable storage medium includes a head disk assembly and electronics to control operation of the head disk assembly. The head disk assembly can include one or more disks. In a magnetic disk drive, a disk includes a recording surface to receive and store user information. The recording surface can be constructed of a substrate of metal, ceramic, glass or plastic with a very thin magnetizable layer on either side of the substrate. Data is transferred to and from the recording surface via a head mounted on an actuator assembly. Heads can include one or more read and/or write elements, or read/write elements, for reading and/or writing data. Drives can include one or more heads for reading and/or writing. In magnetic disk drives, heads can include a thin film inductive write element and a magneto-resistive read element.
Disk drives can operate in one or more modes or operations. In a first mode or operation, often referred to as seek or seeking, a head moves from its current location, across a disk surface to a selected track. In a second mode, often referred to as track following, a head is positioned over a selected track for reading data from a track or writing data to a track.
In order to move a head to a selected track or to position a head over selected tracks for writing and reading, servo control electronics are used. In some disk drives, one disk can be dedicated to servo information. The servo disk can have embedded servo patterns that are read by a head. Heads for data disks can be coupled to the servo disk head to be accurately positioned over selected tracks. In other disk drives, servo information can be embedded within tracks that also contain user data on the medium at regular intervals. Servo information is read as a head passes over a track to accurately position the head relative to a track.
While servo positioning circuitry is generally accurate, heads can drift from desired locations during track following operations. Reading or writing data during inaccurate head positioning can have adverse affects on drive performance.
During write and read operations, the drive attempts to keep the appropriate head or element as close to the center of a selected data track as possible. Data written while the write element is positioned away from a track centerline can later be difficult to read, possibly resulting in transfer errors. Furthermore, data written away from a track centerline can corrupt data on other tracks as well as interfere with reading of data on other tracks.
In modern disk drives, tracks are placed increasingly closer together to increase data storage capacity. Narrower tracks are often used in order to increase the tracks per inch (TPI) on a disk. Measures can be used in drives to ensure that reliability and performance are maintained as data storage capacity increases.